1. Field of the Invention
The present invention relates to a radiation diaphragm (stop) apparatus for accurately forming a radiation field which is an area to be exposed to radiation for use in apparatus adapted for radiotherapy or radiation-based nondestructive inspection and a radiotherapy apparatus equipped with the radiation diaphragm apparatus.
2. Description of the Related Art
The radiotherapy apparatus is one which irradiates a given region containing a diseased part with ionizing radiation to destroy diseased tissues, thereby allowing treatment of the diseased part. Treatments using this apparatus include postoperation treatment, under-operation treatment, noninvasive treatment, etc. The purpose of the postoperation treatment is to, after a surgical operation has been performed on a patient to remove a malignant tumor in a diseased part, externally apply radiation to the diseased part to destroy the remaining tumor cells the operation has failed to remove. The purpose of the under-operation treatment is to directly apply radiation to unremoved tumor cells while the diseased part is cut open. The purpose of the noninvasive treatment is to apply radiation to a diseased part of a patient without opening the diseased part. Advances in computer technology and medical technology have allowed the recent radiotherapy apparatus to irradiate an object of treatment with radiation in large doses, but irradiate surrounding normal tissues with as little radiation as possible. Thus, the radiotherapy is receiving attention as a treatment that has few side effects and is little invasive (or noninvasive).
With such a radiotherapy apparatus, because of its property of irradiating a diseased part with radiation, provisions are made for reducing exposure of a patient to radiation. One of the provisions is to equip the apparatus with a split diaphragm apparatus for restricting the radiation field so that radiation is accurately applied to the diseased part (region for treatment) (see, for example, Japanese Unexamined Patent Publications Nos. 2004-275243 and 2002-210026).
FIG. 1 is a diagram for use in explanation of an example of a conventional split diaphragm apparatus. As shown, first diaphragm elements 100A and second diaphragm 100B are arranged along one direction (X-axis direction in the diagram). The first and second diaphragm elements are configured to be movable in the Y-axis direction normal to the X-axis direction (that is, the first and second diaphragm elements facing each other can be moved to approach each other or separate from each other). By moving each diaphragm element to a desired position, a radiation field of a desired shape can be formed.
Each diaphragm element is formed of a heavy metal, such as tungsten, which is capable of blocking radiation. Therefore, its weight is heavy and the supporting mechanism requires accommodations. The conventional supporting mechanisms include one which supports each diaphragm block with a grooved roller (see, for example, Japanese Unexamined Patent Publication No. 6-210012 and Japanese Patent Application No. 63-267324), one in which each diaphragm element is formed with grooves of V (U)-shaped cross section on both sides, balls and retainers are placed in the grooves, and springs are placed at both ends of each of the grooves to thereby allow adjacent diaphragm blocks to support each other (see, for example, Japanese Unexamined Patent Publication No. 2001-066397), etc.
However, with the conventional radiotherapy apparatus, there are the following problems.
First, with the method of supporting each diaphragm element with a grooved roller, rolling contact results in the radial direction while slide friction results in the thrust direction. As the result, there arises a possibility that reliable accuracy may drop with time due to abrasion. In addition, since the diaphragm block is made of a heavy metal such as tungsten, the grooved roller must be increased in diameter. Thus, the diaphragm apparatus will increase in size to project in the direction of a patient, which will intensify the patient's oppressive feeling. Moreover, it becomes difficult to secure space sufficient to accommodate a standard auxiliary apparatus, such as a block tray, between the radiation source and the patient as needed. Furthermore, when the diaphragm block is subdivided, three or more grooved rollers must be placed for each diaphragm block. Therefore, a large number of rollers must be placed, taking up much space. Further, assembly and adjustment also become difficult.
With the method of using grooves of V (U)-shaped cross section formed on both the sides of each diaphragm element, the structure requires a large number of parts and a large number of steps of assembly. Thus, the cost increases. In addition, because of rolling contact, the driving power is low and looseness is liable to occur.